The researchers examined the by-products of ancient methane-using bacteria in sediments of the Santa Barbara Basin, off the coast of California. They found evidence that methane trapped in ice crystals (known as methane hydrates) on the seabed was released into the water 44,000 years ago - at the same time there was a rapid, but as yet unexplained, rise in global temperatures.

Methane is a potent greenhouse gas, about 10 times more powerful than carbon dioxide. Scientists estimate that approximately 3,000 times the volume of methane in the atmosphere is currently trapped in hydrates at the bottom of the sea.

While conclusive proof is still lacking that the seabed methane released into ocean sediments 44,000 years ago reached the atmosphere, the fact that the hydrates are not as stable as scientists believed raises the possibility that they could be a climatic time-bomb in the future.

Methane hydrates are formed under conditions of low temperatures and high pressures. The methane release of long ago might have resulted from large-scale landslides, or a sea temperatures rising and melting the ice that traps the methane. Since their discovery, scientists have been investigating them as a future source of energy.

"It looks like the destabilisation of the hydrates could have been triggered by the warming that occurred about that time," said Dr David Ethridge, of CSIRO Atmospheric Research, a specialist division of Australia's Commonwealth Scientific & Industrial Research Organisation.

While other factors, such as rotting wetlands, can trigger a rise in atmospheric methane, destabilisation of methane hydrates would lead to a very large spike of methane in the atmosphere and an enormous increase in the rate of warming, Ethridge told ABC Science Online.

The methane pulse released in the incident 44,000 years ago was about 90 million tonnes. Today, this would represent nearly 20% of the annual emissions of methane from all sources on Earth and about a third of the emissions from human activity.

"If you go into the climate world and ask for a prediction of climate for the next 100 years, people can give you a range of estimates for warming," Ethridge said.

"But then people often talk about surprises - things that we may not have understood which could happen - this might be one of those," he added. "It has a low chance of happening but a high impact if it does."